Regarding thin films containing europium, thin films obtained by adding europium to a gate insulating film or adding europium to an optical material, are known (see, for example, Patent Literature 1 and Non-Patent Literature 1).
Regarding the precursor that is used when these europium-containing thin films are formed by a chemical vapor deposition method (hereinafter, abbreviated to CVD method) or an atomic layer deposition method (hereinafter, abbreviated to ALD method), trisdipivaloylmethanatoeuropium (hereinafter, abbreviated to Eu(dpm)3) and europium fluorinated β-diketonate (hereinafter, abbreviated to Eu(hfac)3) are known (see Non-Patent Literature 2).
Furthermore, Patent Literature 1 describes film formation using tricyclopentadienyleuropium (hereinafter, abbreviated to Eu(Cp)3), which is a trivalent cyclopentadienyl-based compound, or Eu[C5(CH3)5]2, which is a divalent cyclopentadienyl-based compound.